1. Field of the Invention
The present invention relates to an image pickup apparatus, or more particularly, to an image pickup apparatus that is preferably adapted to an electronic-zooming image pickup system that employs an image pickup device.
2. Description of the Related Art
In recent years, electronic image pickup apparatuses including an electronic still camera have become popular. In the electronic image pickup apparatus, a photoelectric converting element such as a charge-coupled device (CCD) converts a subject image into an electric signal. Predetermined digital signal processing is performed on the signal, and the resultant signal is recorded on a recording medium such as a memory card. In conformity with the recent trend toward diversification, some types of electronic image pickup apparatuses include an electronic zooming facility capable of optically or electronically enlarging or reducing a subject image.
The electronic zooming facility performs image processing on a recording image signal that can be recorded using an electronic camera or an image signal recorded on a recording medium so as to electrically enlarge part of an image. The enlarged image can be displayed in a whole screen field on the display screen of an image displaying means.
Using the facility, aside from an image of a predetermined image-capturing range defined by a photographing optical system included in the electronic camera, an image produced by enlarging part of the image can be captured (picked up and recorded) or reproduced.
In recent years, there has been a marked increase in the number of pixels capable of being produced by a solid-state image pickup device that is designed for producing a still image. If the solid-state image pickup device designed for producing a still image is applied to production of a motion picture, it takes too much time to read all pixels. For this reason, the solid-state image pickup device may be scanned with the pixels produced thereby skip-read out, so that the number of acquired pixels will match an image size offered by a displaying system.
Furthermore, Japanese Unexamined Patent Application Publication No. 2000-295530 has disclosed a method for electronically zooming an image by changing a pixel skip-reading-out rate. FIG. 16A and FIG. 16B are explanatory diagrams concerning reading proposed in the publication. FIG. 16A is concerned with reading in a normal mode, while FIG. 16B is concerned with reading in a magnification-2 zoom mode.
In the normal mode, the solid-state image pickup device is scanned to read every other pixel. For example, a signal to be read represents pixels depicted with squares painted in black within a range defined with bold-lines in FIG. 16A. specifically, reading is performed at a pixel skip-reading-out rate of 2, that is, a signal representing a matrix of pixels arrayed in four rows and four columns is read from a range covering pixels arrayed in eight rows and eight columns. Thus, the number of read pixels is diminished in order to shorten a reading time and a signal processing time.
In contrast, as shown in FIG. 16B, when the zoom mode is designated, a matrix of consecutive pixels arrayed in four rows and four columns (part of FIG. 16B painted in black) is read from the center of the range that contains a matrix of pixels arrayed in eight rows and eight columns and that is defined with bold lines. Namely, in this case, reading is performed at a skip-reading-out rate of 1. Consequently, the center of the range defined with the bold lines can be displayed in enlargement. Moreover, the number of pixels to be read is the same as that in the normal mode. The same reading time and signal processing time as those required in the normal mode are spent. Thus, the same signal processing can be shared between the normal mode and zoom mode.
By the way, when the solid-state image pickup device is used to sample an image, an aliasing noise is produced. The aliasing noise is visualized as, for example, a moiré and deteriorates the image quality terribly. Normally, an optical low-pass filter is included for minimizing the aliasing noise.
An optical low-pass filter making a modulation transfer function (MTF) zero at a sample rate at which the image is sampled is generally selected considering the characteristic of the low-pass filter.
However, the optical low-pass filter is not designed to cope with pixel skip-reading-out. During pixel skip-reading-out, the sample rate is low. An aliasing noise of a relatively high level is superposed in a low-frequency component, whereby image quality deteriorates terribly.